1. Field of the Invention
The present invention generally relates to a fuel cell, and more particularly to a bipolar plate of a fuel cell.
2. Description of the Related Art
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass from anode to cathode of the fuel cell. In many fuel cells, anode and/or cathode comprise a layer of electrically conductive, catalytically active particles, usually in a polymeric hydrophobic binder such as polytetrafluoroethylene. (PTFE), on the proton exchange membrane. Alternatively, the anode and the cathode layers are applied to the gas diffusion structure. The gas diffusion structure allows the entry of fuel or oxidant to the cell. The gas diffusion/electrode structure, such as carbon cloth or paper material, is pressed to the membrane. The gas diffusion/electrode structure provides the functions that hydrogen (fuel) is effectively transported to the anode catalyst and that oxygen (oxidant) is effectively transported to the cathode catalyst. The resulting structure consisting of the membrane, electrodes and optional gas diffusion structure is referred to as a membrane electrode assembly (MEA).
At the anode, hydrogen molecule (fuel) is oxidized to produce hydrogen protons that will pass through the PEM. The electrons released from hydrogen travel through the external circuitry to do work. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations:
H2 → 2H+ + 2e−at the anode of the cell,andO2 + 4H+ + 4e− → 2H2Oat the cathode of the cell.
Because a single fuel cell typically produces a relatively low voltage (less than 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage.
In the arrangement of the fuel cell stack, bipolar plates are provided between adjacent cells. The bipolar plates may be made from graphite or metal for isolating the reactants, e.g. hydrogen and oxygen and, conducting the electricity from one side to the other. Bipolar plates may include various flow channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
However, bipolar plate made of graphite is fragile and in practice, has to have considerable thickness for supporting the fuel cell stack. The density of graphite is relatively high, so the bipolar plate is heavy and is about 70 percent of the cell stack in weight. Besides, the bipolar plate made of graphite is hard to be mass-produced and expensive. The graphite bipolar plate is about 60 percent of the fuel cell stack in cost.
Due to the weight and the cost of the graphite plate, the metal bipolar plate is developed. However, metal bipolar plate will be oxidized gradually, so the surface resistance between the bipolar plate and the gas diffusion structure is increased. Besides, metal bipolar may release metal ions which will poison the MEA.
Furthermore, the bipolar plate made of plastic mixed with the graphite or carbon fibers is developed. This bipolar plate has to be formed by compression or injection molding process and its conductivity is relatively lower.
The bipolar plates are stacked to form the fuel cell stack, and are compressed to reduce the contact or constriction resistance between them and the gas diffusion structure. Therefore, the bipolar plates must be provided with a peripheral portion around the flow channel field for assembling the bipolar plates into the fuel cell stack and supporting the weight of the fuel cell stack as well as providing the compressional force.
As mentioned above, the hydrogen protons are generated at the anode in the hydrogen fuel cell, and migrate to the cathode through the PEM. The PEM will dry out due to the migration of the protons, and thus be degraded. An auxiliary device, such as pumps and pipes, is provided to feed water into the anode. Also, the water is generated at cathode, and must be removed to avoid the flood of the gas diffusion/electrode structure which hinders the reaction between the oxidant and the catalyst.
Accordingly, there exists a need for a bipolar plate in a fuel cell which is made by lightweight material and provided with a water transmitting device so as to solve the above mentioned problems and disadvantages.